Semiconductors of the group III nitrides, for example, GaN (gallium nitride) based semiconductors are expected as the material of next-generation power semiconductor devices. Compared with Si (silicon), GaN based semiconductor devices have a wider band gap and can realize, compared with semiconductor devices of Si, higher breakdown voltages and lower losses.
An HEMT (High Electron Mobility Transistor) structure using a two-dimensional electron gas (2DEG) as a carrier is generally applied to GaN based transistors. Ordinary HEMT becomes a normally on transistor that allows conduction without a voltage being applied to the gate. Thus, a problem is posed that it is difficult to realize a normally off transistor that does not allow conduction unless a voltage is applied to the gate.
A normally off operation is required of power supply circuits dealing with power of several hundred to one thousand volts from the viewpoint of safety. Thus, a circuit configuration that realizes a normally off operation by cascode-connecting a normally on GaN based transistor and a normally off Si transistor is proposed.
In an inverter circuit of a control system to which a motor as an inductive load is connected, a reflux current from the motor flows to a switching element when the switching element is turned off. If the above circuit configuration is applied to the switching element of an inverter circuit of a motor control system, a reflux current flows to a body diode of a normally off Si transistor. Due to inferior recovery characteristics of the body diode of the Si transistor, power loss of the inverter circuit may increase when the reflux current flows.